Grade 07 Science Unit 02 | Static Electricity Lesson Short Notes Term Test Questions Answers Exercise Activity Experiments Take advantage of this free resource and start preparing Nalanda College

Here's a detailed explanation of each key point related to Static Electricity based on the provided content:

02. Static Electricity

Static electricity is the accumulation of electric charges on the surface of an object due to friction, leading to attraction or repulsion between objects.

2.1 Charging an Object

• When a plastic pen is rubbed against dry hair and brought close to small pieces of paper, the pieces of paper are attracted to the pen.

• Other objects, such as PVC pipes or glass rods, can also attract pieces of paper if rubbed with specific materials.

Why Does This Happen?

• Rubbing certain materials together transfers electric charges between them.

• This process is known as charging by friction.

• The scientist William Gilbert (1600 AD) was the first to discover that rubbing objects (e.g., amber with silk) generates electric charges.

Activity 2.1: Observing Static Electricity

Materials Required

• Glass rod

• PVC pipe

• Ebonite rod

• Sheet of polythene

• Silk cloth

• Woolen cloth

• Small pieces of paper

Procedure

1. Hold each object close to small pieces of paper and observe if they attract the paper before rubbing.

2. Rub each object with the specified material: Glass rod with silk cloth PVC pipe with polythene sheet Ebonite rod with woolen cloth

3. Again, hold the objects near the paper and observe whether they now attract the paper.

Observations

• Before rubbing → No attraction

• After rubbing → Objects attract the paper

• Conclusion → Rubbing transfers electric charges, making the object capable of attracting light materials.

Scientific Explanation

• Rubbing an object transfers electric charges to its surface.

• These charges are called static electric charges.

2.2 Types of Static Electric Charges

Static electricity consists of two types of electric charges:

1. Positive (+) Static Electric Charges

2. Negative (-) Static Electric Charges

Activity 2.2: Studying Static Electric Charges

Materials Required

• Two glass rods

• Two ebonite rods

• Silk cloth

• Woolen cloth

• Thread

• Two stands

Procedure

1. Hang a glass rod rubbed with silk from one stand.

2. Hang an ebonite rod rubbed with wool from the other stand.

3. Charge another glass rod by rubbing it with silk and bring it close to the hanging rods.

4. Charge another ebonite rod by rubbing it with wool and bring it close to the hanging rods.

Observations

• Glass rod vs. Glass rod → Repulsion

• Glass rod vs. Ebonite rod → Attraction

• Ebonite rod vs. Ebonite rod → Repulsion

Explanation

• Objects with like charges repel each other.

• Objects with unlike charges attract each other.

• A glass rod rubbed with silk gains positive (+) charges.

• An ebonite rod rubbed with wool gains negative (-) charges.

Exercise

• A PVC rod rubbed with polythene is attracted to a glass rod rubbed with silk.

• Since glass gains positive charges, the PVC rod must have negative charges.

2.3 Generation of Static Electric Charges

• Objects initially have equal numbers of positive and negative charges, making them neutral.

• Rubbing transfers electrons (negative charges) between materials.

• The object losing electrons becomes positively charged.

• The object gaining electrons becomes negatively charged.

Illustration of Charge Generation

1. Before Rubbing Objects have equal positive and negative charges → Neutral

2. During Rubbing Electrons are transferred from one object to another.

3. After Rubbing One object is positively charged (loses electrons). One object is negatively charged (gains electrons).

Activity 2.4: Neutralization of Static Charges

Materials Required

• Glass rod

• Ebonite rod

• Silk cloth

• Woolen cloth

• Two stands

• Thread

Procedure

1. Rub the glass rod with silk and the ebonite rod with wool.

2. Hang both rods separately.

3. Bring them close to each other and observe.

4. Repeat multiple times and observe changes.

Observations

• First time: Attraction between the glass and ebonite rods.

• After multiple times: No attraction or repulsion.

Explanation

• When oppositely charged objects touch or come close multiple times, their charges neutralize due to charge exchange.

• Once neutralized, the objects no longer attract or repel.

Summary of Key Concepts

1. Static Electricity is generated by rubbing objects together, transferring electric charges.

2. There are two types of charges: Positive (+) (e.g., glass rod rubbed with silk) Negative (-) (e.g., ebonite rod rubbed with wool)

3. Like charges repel, unlike charges attract.

4. Electrons move from one material to another, creating static charges.

5. Neutralization happens when opposite charges balance out, leading to no net charge.

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Here is a list of possible exam questions and answers based on the topic Static Electricity:

Section 1: Basic Concepts of Static Electricity

1. What is static electricity? 👉 Static electricity is the accumulation of electric charges on the surface of an object, usually caused by friction.

2. Who was the first scientist to study static electricity? 👉 William Gilbert, in 1600 AD, was the first scientist to study static electricity.

3. What happens when a plastic pen is rubbed against dry hair and brought close to small pieces of paper? 👉 The pieces of paper are attracted to the pen due to the static electric charges generated on its surface.

4. What are static electric charges? 👉 Static electric charges are electric charges generated on the surface of an object due to rubbing.

Section 2: Charging an Object

5. What is the process of charging an object by rubbing called? 👉 It is called triboelectric charging or frictional charging.

6. Why do objects attract light materials like paper after being rubbed? 👉 When an object is rubbed, it gains electric charges that attract neutral light materials like pieces of paper.

7. Name three materials that can be used to create static electricity by rubbing. 👉 Glass rod, PVC pipe, ebonite rod.

8. What happens when you rub a glass rod with silk? 👉 The glass rod becomes positively charged as it loses electrons.

9. What happens when you rub an ebonite rod with wool? 👉 The ebonite rod becomes negatively charged as it gains electrons.

Section 3: Types of Static Electric Charges

10. What are the two types of static electric charges? 👉 Positive (+) charge and Negative (-) charge.

11. What is the rule of attraction and repulsion in static electricity? 👉 - Like charges repel each other 👉 - Unlike charges attract each other

12. What happens when you bring two glass rods rubbed with silk close to each other? 👉 They repel each other because both are positively charged.

13. What happens when you bring an ebonite rod rubbed with wool close to a glass rod rubbed with silk? 👉 They attract each other because they have opposite charges.

Section 4: Generation of Static Electric Charges

14. Why does an object remain neutral before rubbing? 👉 Because it has an equal number of positive and negative charges.

15. How does rubbing generate static electricity? 👉 Rubbing transfers electrons from one object to another, creating a charge imbalance.

16. What charge does an object get when it loses electrons? 👉 It becomes positively charged.

17. What charge does an object get when it gains electrons? 👉 It becomes negatively charged.

Section 5: Practical Applications and Experiments

18. What is the purpose of a gold leaf electroscope? 👉 It is used to detect static electric charges.

19. What is observed when a charged object is brought near a gold leaf electroscope? 👉 The gold leaves diverge (spread apart) due to repulsion of like charges.

20. What happens when a negatively charged object is brought near a positively charged object? 👉 They attract each other.

21. How can a charged object be neutralized? 👉 By bringing it into contact with an oppositely charged object or grounding it.

Section 6: Experimental Observations

22. What happens when a charged straw is brought close to another similarly charged straw? 👉 They repel each other.

23. What happens when a charged straw is brought close to a piece of polythene used to charge it? 👉 They attract each other because they have opposite charges.

24. What happens when a positively charged and negatively charged rod are brought close to each other multiple times? 👉 They attract at first but become neutralized after repeated contact.

Section 7: Conceptual and Application-Based Questions

25. Why do you sometimes get a small electric shock when touching a metal object after walking on a carpet? 👉 Because static electricity builds up in your body due to friction, and when you touch a conductor, the charge discharges suddenly.

26. Why do clothes stick together after being taken out of a dryer? 👉 Due to static electricity, where some clothes gain positive charges and others gain negative charges, causing attraction.

27. Why does rubbing a balloon on your hair make it stick to the wall? 👉 The balloon gets negatively charged and attracts the positively charged surface of the wall due to static electricity.

28. If a PVC rod rubbed with polythene is attracted to a glass rod rubbed with silk, what is the charge on the PVC rod? 👉 The PVC rod is negatively charged.

29. How can you prevent static electricity buildup in everyday life? 👉 - Use humidifiers to reduce static buildup. 👉 - Wear cotton clothes instead of synthetic materials. 👉 - Use anti-static sprays or lotions.

These questions cover theoretical concepts, observations, experiments, and real-life applications of static electricity. Let me know if you need more questions or explanations! 🚀

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